1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, to a method and apparatus for improving the performance of such systems and the portable units cooperating therewith.
2. Discussion of the Background Art
Wireless communication systems conventionally comprise one or more base stations, a plurality of mobile units, and a plurality of portable units. The term xe2x80x9cmobile unitxe2x80x9d should be understood as referring to a communication device located within a vehicle and powered by a battery located in the vehicle, while a xe2x80x9cportable unitxe2x80x9d is intended to refer to a communication device carried by a person and powered by a smaller portable battery (typically unitary with the communication device). A typical mobile unit is operative to transmit RF communication signals at a power level on the order of about 20 Watts, while a typical portable unit is operative to transmit such signals at a power level of about 3 Watts.
Wireless communication systems operate with each base station being fixed at a single geographic location and configured to transmit high powered RF energy from an antenna mounted at a significant height above ground level. Similarly, each base station receives RF energy at an antenna mounted at a similar significant height above ground level. A base station communicates with one or more portable units that are geographically positioned within some radius defined by minimum power level necessary to successfully receive an RF signal transmitted to and from the base station. Communication between a base station and a central office is established over a land based cable network or, alternatively, by a microwave point to point link using a frequency range different from that used for communication between a base station and the mobile units. To allow a plurality of base stations to inter-operate as a single wireless system, the central office performs various switching and routing functions.
Portable units are typically hand held by the wireless system user and operate to communicate with one of the base stations to provide connectivity to the larger wireless system. Ideally, a user with one of the portable units is able to maintain communication anywhere within a defined geographic region. While an indicator of the quality of a wireless communication system is the percentage of coverage available within a given geographic area, it is a practical reality that natural and artificial topographies create communication xe2x80x9cshadowsxe2x80x9d where communication between a portable unit and a base station will degrade or cease altogether. A mobile unit, however, may still be within the coverage area supported by the base station as a result of its higher powered RF transmission capacity. The size and number of the shadows reduce the percentage of portable coverage within a given geographic area.
One possible solution to the shadow problem is to erect one or more additional base stations that are operable to transmit and receive within a respective shadowed area. As with any increase in infrastructure, however, an election to include additional base stations would significantly increase the overall system cost Moreover, the shadowed areas may be used only on an infrequent basis such that the cost and maintenance of additional base stations would not be warranted. An alternate solution to the shadow problem requiring a substantially lower capital investment is to add a component to the overall wireless system called a mobile vehicular repeater unit. The mobile vehicular repeater unit operates as a movable base station that is, for example, permanently mounted in a vehicle driven by one or more portable unit users. The mobile vehicular repeater unit acts as an intermediary between the portable unit and the base station to relay information received from the portable unit to the base station and vice versa. The mobile vehicular repeater unit, in practical effect therefore, becomes a xe2x80x9croaming base stationxe2x80x9d by receiving communications from the base station and further transmitting those communications to one or more portable units.
Each radio channel employed in a wireless transmissions system having mobile repeater units and portable units typically consists of two frequencies which are separated by a fixed frequency offset. In such instances the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) has defined the forward channel portion, i.e. base station transmission to the portable unit/mobile unit, as operating at the higher frequency of the pair. The FCC has further defined the reverse channel portion, i.e. base station reception from the portable unit/mobile unit, as operating at the lower frequency of the pair. A disadvantage arises, however, in that the channels assigned to a given mobile vehicular repeater unit and associated portable units may change from region to region. While the vehicular repeater may, for example, incorporate a global positioning system (GPS) and/or communicate with a centralized or regional facility to learn which channels to use, portable subscriber units are not likely to be equipped with GPS and may also be unable to communicate with the ROC to learn which channels to use. Accordingly, there exists a need to reliably provide the portable units with frequency channel information.
The aforementioned need is addressed, and an advance is made in the art, by a communication system comprising a base station, a mobile vehicular repeater unit, and one or more portable units in which the mobile vehicular repeater unit is operable to relay frequency channel assignment information to the portable units in response to a wake-up request transmitted by a first of the one or more portable units.
Where the portable unit has coverage between at least one of the base stations and the vehicular repeater over an assigned frequency, the base station transmits to the mobile vehicular repeater unit over a first or forward frequency channel portion, while the mobile vehicular repeater unit transmits to the base station over a second or reverse frequency channel portion. During such operation, an assigned frequency channel portion is used for transmission from either of the mobile vehicular repeater unit and the portable unit to the other.
In the event a portable unit enters an area in which the currently assigned transmit and receive frequency no longer provides coverage to the base station, that portable unit is operable to transmit a channel access request to any nearby mobile vehicular repeater unitxe2x80x94a responding repeater unit being operative to obtain a new operating frequency from, for example, a centralized facility, and to transmit a signal representative of the newly assigned frequency channel to the portable unit. The portable unit is operative to acknowledge receipt of the updated information via the newly assigned frequency channel, whereupon both the mobile vehicular repeater unit and the portable unit begin transmitting using the newly assigned frequency channel.
A method for communicating in accordance with the present invention comprises the steps of a portable unit following a frequency use plan of transmitting to and receiving signals from a mobile vehicular repeater unit or a base station on a respective assigned frequency channel portion. The process further comprises attempting to establish communication between the portable unit and a base station during or after a change in geographic location. If the attempt to establish communication between the portable unit and a base station fails, the portable unit synchronizes to a known beacon frequency over which each mobile vehicular repeater unit periodically announces its availability. Over a reverse channel portion of the beacon frequency, the portable unit transmits a request for the vehicular repeater to identify an appropriate frequency pair over which they may exchange RF transmission signals and thereby provide the portable unit with connectivity to the base station. In accordance with another aspect of the inventive process, the mobile vehicular repeater unit responds by obtaining and providing the frequency pair identification to the first requesting portable unit and, thereafter, it periodically transmits blocks of bits representative of the frequency channel identification for use by any subsequent portable units synchronizing to the beacon frequency that may be used by the mobile vehicular repeater unit and portable unit and so as to restore or provide connectivity to the base station via the mobile vehicular repeater or indirect in accordance with an extended coverage mode of operation.